The invention relates to a method of manufacturing solid glass preforms for the manufacture of optical components. The solid preforms are manufactured by collapsing hollow preforms having a refractive index which varies across the wall thickness to achieve desired properties. The hollow preform is collapsed by moving a heating zone along the preform. The length of the heating zone is smaller than the length of the hollow preform.
The hollow preform collapses due to the surface tension of the heated glass and possible pressure differences. In this connection glass is to be understood to include also quartz glass.
Optical fibers can be drawn from solid preforms obtained by the method according to the invention. Lenses can be manufactured from solid preforms having graded refractive index distributions by dividing the preform into wafers of suitable thickness.
The hollow preforms may consist, for example, of quartz tubes. The tubes are coated on their insides with one or more glass layers of suitable refractive indices as desired for the optical components to be manufactured from the solid preform. The refractive index may increase in one or more steps or continuously from the outside to the inside. Hollow preforms may alternatively be obtained by providing glass particles on a mandril in layers having refractive indices which vary as desired.
Methods of manufacturing hollow preforms are known per se. Several such methods are described by G. J. Koel in an article entitled "Technical and economic aspects of the different fibre fabrication processes" (Proceedings of the 8th European Conference on Optical Communication, Cannes, September 1982, pp. 1-8).
The collapsing temperature for internally coated quartz tubes is between approximately 1900.degree. and 2200.degree. C. Generally, hydrogen-oxygen burners are used for collapsing internally coated quartz tubes. In practice, the temperature of a flame of such a burner usually is not much more than 2500.degree. C. As a result, the heat flux is comparatively low and the time required to completely collapse the tube to form a solid preform is rather long, particularly if the hollow preforms have large diameters. The heat transfer takes place by transfer of kinetic energy of gas molecules to the glass.
A comparatively long collapse time requires measures to suppress the evaporation of comparatively volatile constituents from the innermost glass layers or to remove the disadvantages thereof (for example by etching away, immediately prior to the closing of the cavity in the preform, a layer from which the volatile constituent has been evaporated). However, these measures are difficult to control and they require much effort to obtain reproducible results.
It is furthermore of great importance that during the collapsing process a good geometry is maintained. During the collapsing process acentricity, bending or sagging of the preform should be avoided. This is of importance in particular if the solid preform is again inserted into a quartz tube and together with the latter is drawn into an optical fiber.
Nonuniform heating of the hollow preform and a nonuniform pressure on the exterior of the preform must therefore be prevented as much as possible during the collapsing process.